In-ear earphones or earbuds continue to be popular since they can deliver reasonable sound quality while having a conveniently small profile and being lightweight. Professional quality in-ear earphones often use balanced armature drivers that can be designed to faithfully reproduce either low frequency sound or high frequency sound. However, balanced armature drivers generally do not operate consistently across the entire audible frequency range. To overcome this limitation, multiple balanced armature drivers have been suggested for within an in-ear earphone. A crossover network is also provided in that case, to divide the frequency spectrum of an audio signal into two regions, that is, low and high, and a separate driver is used to reproduce the sound in each region. Professional quality earphones may also have an ear tip or sleeve, which can be either custom molded or generic, that allows for a snug fit that is intended to acoustically seal against the ear canal of the user, which enables a higher quality low frequency or bass sound to be heard, in addition to lower acoustic background noise.
A typical sealing-type earbud has a housing or cup in which a driver is housed. A silicone or rubber boot that has sound passages formed therein fits over the front of the driver, to hold the driver in place, and to ensure that the driver output is sealed relative to the outside environment. A cap that is made of a rigid material (in contrast to the material of the boot) is then pushed onto the boot to essentially complete a rigid earphone housing. A spout extends out the front of the cap, and is aligned with the passages in the boot so as to receive the sound produced by the drivers. A flexible ear tip is then fitted to the spout. While this arrangement has proven to be effective in terms of presenting reasonable sound performance while being sufficiently small and light enough for everyday consumers use with various activities, a generic, that is a non-custom, in-ear earphone that is suitable for high volume manufacture that provides good sound fidelity across most, if not all, of the audible frequency range of a typical consumer presents a challenge, particularly in terms of packaging multiple drivers inside the tight confines of the earbud housing.